The traditional technique of wet-chemical metallisation consists of etching the surface of non-conductive polymers with chromic acid solutions, catalytic activation with a solution comprising a noble metal, and the wet-chemical metallisation to obtain a first metallic layer, usually a first layer of copper or nickel (alloys).
Etching is required to provide hydrophilic properties to the surface of the non-conductive polymers which are important to adsorb sufficient amounts of noble metal from the aqueous activator solution and ensure a good adhesion between the first metallic layer and the non-conductive polymer.
Activation with a solution comprising a noble metal is carried out in order to start wet-chemical deposition of a first metal or metal alloy layer onto the surface of the non-conductive polymer. This first metallic layer then serves as the plating base for depositing one or more metallic layers thereon by electroplating.
The main disadvantages of the traditional technique is the carcinogenicity of chromic acid (hexavalent chromium) used for etching the surface of the non-conductive polymer.
The US application 2005/0199587 A1 discloses an acidic etching solution for non-conductive polymers comprising 20 to 70 g/l potassium permanganate. The optimum concentration of potassium permanganate is about 50 g/l. When the permanganate ion concentration is lower than 20 g/l the solution is ineffective, whereas the upper margin of concentration is limited by the solubility of potassium permanganate. After etching, the activation is carried out with palladium salt solutions containing amines as complexing agents and later the non-conductive polymer is processed with a reducing agent, e.g. a borohydride, a hypophosphite or a hydrazine solution.
In case of a high permanganate concentration in the etching solution (recommended about 50 g/l and about 40 vol.-% of sulfuric or phosphoric acid), permanganate decomposes easily especially at an elevated temperature of e.g. 37° C. At this temperature, the etching solution becomes ineffective after 4 to 6 h. The wettability of the non-conductive polymer then is not high enough for adsorbing a sufficient amount of noble metal from the aqueous activator solution. Furthermore, the adhesion between non-conductive polymer and plated metallic layer(s) decreases significantly. In addition, the insoluble permanganate dissociation products are enriched in the etching solution and can contaminate the surface to be plated.
The patent LT 5645 B discloses a method for applying a first metallic layer onto a non-conductive polymer. The aqueous etching solution applied in the first step contains 70 to 90 wt.-% sulfuric acid and 0.0001 to 0.01 wt.-% (0.001 to 0.1 g/l) permanganate. An ionogenic activator solution comprising palladium ions and urea is applied in the second step followed by wet-chemical deposition of a first metallic layer.
An etching solution for poly(aryl-ether-ether-ketone) (PEEK), the solution comprising 1 wt.-% (10 g/l) potassium permanganate, five volume parts (56 vol.-%) of concentrated sulfuric acid, two volume parts of 85% orthophosphoric acid and two volume parts of water is disclosed in the article “Permanganic etching of PEEK” (R. H. Olley, D. C. Bassett, D. J. Blundell, Polymers, 27: 344-348, 1986). However, neither an activation step nor the deposition of a first metallic layer thereon is disclosed in this document. This etching solution was used in Example 4 (comparative) of the present invention.
A method for metallization of a polymer surface is disclosed in in the patent application EP 1 001 052 A2. The polymer surface is etched with a “mild” acidic solution comprising an oxidant. Next, the surface is activated with an aqueous solution of a metal salt selected from cobalt-, silver-, tin and lead salts followed by contacting the surface with a sulfide solution and deposition of a first metallic layer.